1. Field of the Invention
This invention relates to a semi-hot forging (hereinafter referred to as warm forging) method for forming a cup-shaped piece by using a relatively thin blank, the forged cup-shaped piece having a open end with a greater sectional area than that of the blank. More particularly, the invention provides such a warm forging method for a cup-shaped piece according to which the blank is subjected to a warm treatment and upset in the form of the bulged shape in an open forging die with both ends of said blank being clamped, and then a punch is press-fitted thereinto to extrude the forged piece backwardly.
This invention also relates to a method intended to enable a smooth backward extrusion forging operation by effectively performing lubrication and cooling of the punch which directly participates in the backward extruding operation.
2. Description of the Prior Art
Generally, for producing a relatively shallow cup-shaped piece such as a chain roller, bearing race, nut, etc., or a deep cup-shaped hollow and tough piece, a prototype thereof is first forged, then cut off, drilled and further worked and polished into the product. The forged intermediate product is a bottomed cylindrical piece, which is then subjected to drilling, cutting and other work. Such a first forging (intermediate product) is usually cup-shaped, so that it is hereinafter referred to as cup-shaped piece.
Forging of such a cup-shaped piece is usually practiced by hot forging, and in such case there is employed a backward extrusion system using an open forging die. The blank having a sectional area approximately the outer diameter of the cup-shaped piece to be formed is heated and put into the die, then a backward extrusion punch with a size corresponding to the inner diameter of the cup-shaped piece to be formed is press-fitted into the die from its opening to give shape to the peripheral wall and bottom portion. Finally, the forged piece is ejected by means of a knockout pin previously provided at the die bottom. In some cases, upsetting is performed before press-fitting the punch into the die. In such cases, there is employed a piston-like punch so designed as to cover the whole internal peripheral surface of the die, and the backward extrusion punch is press-fitted after said piston-like punch has been upset on the bottom side of the die. Generally, such upsetting and die forging are performed by separate machines, so that the equipment costs are high, and many and complicated steps are required. Also, heating devices are necessitated and such heating makes it difficult to secure high dimensional accuracy. Further, surface roughening tends to take place due to decarburization.
Therefore, employment of low-cost cold forging is being considered by some manufacturers, but such cold forging involves the problems of increased work load and possible failure of the punch member. Also, since the formed piece is work hardened by the working heat caused during compression forming, there is indicated a drawback that it is necessary to perform softening annealing in the course of, or at the final stage of working.
It is an essential requirement, irrespective of hot forging or cold forging, that the forging blanks of the desired lengths be easily obtained by cutting. In case the blanks are rod-shaped and they are sheared and forged by a same forging device, the smaller the rod diameter, the easier becomes the shearing operation and the more accurate becomes the shear plane. However, it was impossible in cold working to achieve backward extrusion, from a blank having a small cross sectional area, so as to form the cup-shaped piece having a greater inner diameter of the hollow portion thereof than an outer diameter of the blank and also a greater cross sectional area of the hollow portion than that of the blank.
There is also known a warm forging system in which the blank is heated to several hundred degrees C. and then forged.